This invention relates to signal compression/expansion apparatus and, more particularly, to such apparatus wherein an input signal is compressed by an exponential factor in the signal compression apparatus; and is expanded by a reciprocal of that exponential factor in the signal expansion apparatus.
In information signal recording, such as in the recording of an audio signal on a recording medium, for example, on a magnetic tape, sheet or disc, or on a record disc, it often is desirable to minimize or effectively eliminate noise. Typically, this noise appears as background noise or hiss. Accordingly, various noise eliminating circuits have been proposed. In a typical noise eliminating circuit now used, the dynamic range of the information signal is narrowed, or reduced, prior to recording, and this range is restored to its original value during reproduction.
In a noise eliminating circuit of the so-called dbx-system, the information signal is compressed by a fractional exponential factor prior to recording and then, during reproduction, the signal is expanded by the reciprocal of that exponential factor. In one embodiment of such a dbx-system, the signal compressing circuitry includes an exponential amplifier, that is, an amplifier whose gain is equal to an exponential function, and also a logarithmic conversion circuit for producing a gain control voltage that is a logarithmic function. The signal expanding circuitry used in the dbx-system also includes an exponential amplifier and a logarithmic conversion circuit.
While the theoretical operation of such a dbx-system may be acceptable, the actual operation of such a dbx-system may be acceptable, the actual operation thereof is less than satisfactory. In particular, exponential amplifiers and logarithmic conversion circuits are complex and, therefore, are expensive to manufacture. Also, it often is difficult to manufacture matched exponential amplifiers which are necessary to achieve signal expansion that is the correct reciprocal of the signal compression. Furthermore, since the circuit components which are used are subjected to a range of tolerances, the difficulty of assembling matched compression and expansion circuits is further increased. Additionally, because of temperature dependencies inherent in many circuit components, even matched circuits may exhibit nonuniform characteristics over a period of time.